Picking characters from a string

[Doug Wolfinger]

Hello,
I am trying to pass a string that represents a postfix expression. The method that accepts it should take each number and place it on the stack. When it encounters an operator, it should pop the top two numbers and evaluate them with the operator.
This is the expression. "abc+-"
a=7, b=3, c=12 So it applies "+" to c and b, then it subtracts (c + b) from a. I have made the string into a string array in my program, but I think the idea is to pass the expression as a regular string as I have it above. Thanks!

class PostFixStack {
    StackArrayBased stack = new StackArrayBased();
    String operator;
    int operand1;
    int operand2;
    int result;          //primitive value which is converted to Integer
    Integer intObj;      //Integer to hold string value
    Integer intOper1;    //used to pop from stack
    Integer intOper2;    //used to pop from stack
    Integer intResult;   //used to convert from result to Integer and push to stack
  public Integer srMeth(String[] str)
  //Accepts a postfix expression as an argument. Processes the expression.
  //Precondition: str.length() > 0.
  //Postcondition: Returns the Integer that results from the expression str.
  {
    for(int i=0;i<str.length;i++)
    {
       if( str[i].equals("*") || str[i].equals("-") || str[i].equals("+") ) {
         operator = str[i];
         intOper1 = (Integer)stack.pop();
         intOper2 = (Integer)stack.pop();
         operand2 = intOper2.intValue();
         operand1 = intOper1.intValue();
         if(operator.equals("*"))
         result = (operand2 * operand1);
         if(operator.equals("+"))
         result = (operand2 + operand1);
         if(operator.equals("-"))
         result = (operand2 - operand1);
         //convert result back to an Integer so that push() will accept it.
         intResult = new Integer(result);  
         stack.push(intResult);
         System.out.println("currently the top value is " + stack.peek());
    }
       //if the array occurrence is a number
       else {
         System.out.println(str[i]);
         intObj = new Integer(str[i]);
         stack.push((Integer)intObj);
       }
     }
       return intResult;      
   }
   public static void main(String args[]) {
     String string[] = {"7","3","12","+","-"};
     String stringB[] = {"7","3","12","-","-5","*","+"};
     PostFixStack pfs = new PostFixStack();
     System.out.println(pfs.srMeth(string) + " = the value of the 9a expression");
     System.out.println(pfs.srMeth(stringB) + " = the value of the 9b expression");
    
     }
    }

If you would like to use that program, you'll need this one, too.

public class StackArrayBased 
                 implements StackInterface {
  final int MAX_STACK = 50;  // maximum size of stack
  private Object items[];
  private int top;
  public StackArrayBased() {
    items = new Object[MAX_STACK]; 
    top = -1; 
  }  // end default constructor
  public boolean isEmpty() {
    return top < 0;
  }  // end isEmpty
  public boolean isFull() {
    return top == MAX_STACK-1;
  }  // end isFull
  public void push(Object newItem) throws StackException {
    if (!isFull()) {
      items[++top] = newItem;
    } 
    else {
      throw new StackException("StackException on " +
                               "push: stack full");
    }  // end if
  }  // end push
  public void popAll() {
    items = new Object[MAX_STACK]; 
    top = -1; 
  }  // end popAll
  
  public Object pop() throws StackException {
    if (!isEmpty()) {
      return items[top--];
    }
    else {
      throw new StackException("StackException on " +
                               "pop: stack empty");
    }  // end if
  }  // end pop
  public Object peek() throws StackException {
    if (!isEmpty()) {
      return items[top];
    }
    else {
      throw new StackException("Stack exception on " +
                               "peek - stack empty");
    }  // end if
  }  // end peek
}  // end StackArrayBased

[Doug Wolfinger]

My bad. I've copied this to the Intermediate forum.